1. Field of the Invention
This invention relates to a distance measuring device for a camera in which a distance measuring light beam is projected on an object (such as a photographic object) whose distance is to be measured, photo-sensitive elements receive light from the object and store electric charges converted from the incident light, the stored quantity of electric charges is detected and a lens is focused on the basis of the electric charge. More particularly, the invention relates to a light receiving arrangement for the distance measuring device.
2. Description of the Prior Art
An example of the photo-sensitive element of the aforesaid electric charge storing type is disclosed in U.S. Pat. No. 3,999,192. Here an electric-charge-storing photo-decoder includes sensors which receive reflected light, as well as a first and second electric charge storages each with two electric-charge-storing element. The photo-detector operates in response to a light projector which projects light through given on-off cycles. First and second control gates are controlled in time with the intermittent cycles of the projected light. Electric charges generated at the sensors when light is projected, and during the intermittent intervals, are then separately stored in corresponding electric charge storing elements of the first and second electric charge storages. The electric charges thus stored at the first and second electric charge storages are drawn out at suitable respective times. In this instance, the pairs of electric charge storing elements in the first and second electric charge storages of corresponding sensors produce electric signals according to the electric charges stored therein. Two differential amplifiers generate the respective differences between these electric signals. The system then detects concordance or discordance between the outputs of the two differential amplifiers and shifts the position of a focusing lens to a focused position on the basis of the detected signal.
With two photo-sensitive elements at a photographic camera producing signals from their photo-electric converters, the differential amplifiers compute and determine the extent to which the focusing lens should be shifted to an in-focus position. Accordingly, the focusing lens can be shifted and stopped at the in-focus point. However, using such differential type amplifiers in a light receiving device requires each of the gates in the light receiving device to be controlled with a high degree of accuracy.
A first subject of the present invention concerns a drive control for a light receiving device using these kinds of differential type amplifiers. A second subject of the invention involves overcoming problems concerning the relationship of the photo-taking lens to a light receiving device of the aforementioned differential amplifier adapted for use in a photographic camera as a focusing device. FIGS. 1-4 of the accompanying drawings illustrate the problems of this second subject.
FIGS. 1 to 4 include an image-forming lens 1 and a light source 3. Light from the light source 3 is projected onto a photographic object 2 through a light projecting lens 4 and the image forming lens 1. The lens 4 and a light receiving lens 5 guide light reflected from the object 2 to photo-electric converting means 6. Photo-electric converting means 6 includes two photo-electric converting elements 6a and 6b. A distance to the object is detected when the two photo-electric converting elements 6a and 6b receive the light reflected from the object 2 equally.
The lens 1 may, for example, form an image of the object to be photographed on a film surface. When an object comes closer, such as to a point 7 as shown in FIG. 2, the resulting focus deviation is mainly toward the element 6b. That is, the image of a spot of light projected onto the object by the lens 1 and returned by the light receiving lens 5 follows the path of the light flux 8. As a result, the photo-electric converting elements 6a and 6b receive different quantities of light. Detection of this difference then indicates that the object is located nearer than an expected object plane. The lens 1 is then shifted toward an in-focus condition.
However, with the position of an object changed, the light striking the photo-electric converting elements 6a and 6b does not always change in exact proportion to the change of the object position. For example, in a zoom lens such as the one shown in FIGS. 3(a) and 3(b), the photo-electric converting means 6 operates correctly when the lens is zoomed to its telephoto end as shown in FIG. 3(a). This is so because the projected light ray 10 does not cross the main received light ray 11 nor a light ray 12 coming to the end of the photo-electric converting means within the lens system of the zoom lens. However, in the case of FIG. 3(b) where the lens is zoomed to its wide angle end, the projected light ray 10 overlaps a light ray 12 at a light receiving end of a lens section 13. The internal reflection by the lens face at the section 13 causes the light to return to the end portion of the photo-sensitive element. The latter does not discriminate internally reflected light from light reflected by the object. This tends to result in an erroneous operation.
Similar inconveniences sometimes arise depending on the object distance or due to shifts in the operation of the focusing lens. Attempts to prevent this by making the light receiving surface of the photo-electric converting means 6 smaller can result in another disadvantage. The reflected light may depart from the light receiving area if the reflected light deviates to any great extent when the lens is zoomed to its telephoto position.
In detecting the difference signal from the paired photo-electric converting elements, increasing the light receiving area enables detection of the signal despite a large deviation. However, the arrangement degrades the deviation detecting accuracy for a delicate change of the light flux near an in-focus point.
FIG. 4 shows an example of the conventional circuit using photo-electric converting elements. This conventional arrangement includes photo-electric converting elements 21 and 22 which correspond to the photo-electric converting elements 6a and 6b and gates 23 and 24 for reading out two photo-electric signals produced by these photo-electric converting elements. These gates are operated via a line 25. The photo-electric conversion values of the elements 21 and 22 are applied to lines 28 and 29 in parallel with additional charges or stored charges 26 and 27. Then, a voltage or electric charge difference between these two lines represents the deviation of the reflected light.